The present invention relates in general to an alkaline cell and more particularly to a separator for the alkaline cell and a method of producing the separator.
Generally known alkaline cells such as LR-6 type alkaline cells have, as illustrated in FIG. 1, a cylindrical casing 2 having a bottom end for serving as a cathode, an electrically conductive membrane (not shown), three cylindrical cathode mixes 3 piled within the cathode cylindrical casing 2, a cylindrical separator 4 having a bottom end, an electrolyte in both the separator 4 and the cathode mixture 3, and an anode active mass 5 of zinc in the form of gel.
The cathode metal casing 2 has an opening to which a closure body 6 is mounted snugly. The sealing body 6 has an anode terminal 7, a current collector 8 fixed to an inner central portion of the anode terminal plate 7 by spot welding and the like, a gasket 9 fitted to the current collector 8 with a sealing material disposed therebetween, and an annular sealing cap 11 firmly fitted to the opening of the cathode casing 2. When the current collector 8 is inserted into the anode active mass 5 of gel-like zinc anode material 5, an end 4A of the separator 4 is pressed inside by the gasket 9 to be bent inside, and while the separator end 4A is bent inside, an end of the opening of the cathode casing 2 is caulked and an outer circumferential portion of the gasket 9 is clamped between the outer circumferential portion of the anode terminal plate 7 and an end of the cathode casing 2, so that the opening of the cathode casing 2 is sealed by the closure body 6.
On an outer surface of the cathode metal casing 2, there is provided a label 12 which has recently replaced the conventional metal jacket for the purpose of increasing the inner volume of the cell and referred to as "shrink tack label" or simply "label", and an annular anode washer 13 is disposed between an outer circumferential portion of the anode terminal plate 7 and the label 12.
With reference to FIG. 2, the separator 4 has been produced by rotating a roll 20 of a base paper 21 for the separator by pulling an end of the roll paper in its longitudinal direction and cut into a predetermined length in accordance with a length (height) of a separator to be formed, and then the base paper 21 of the predetermined length is rotated in the horizontal direction in the drawing by means of a mandrel (not shown) to form a cylindrical structure as illustrated. The cylindrical base paper 21 is then heat-bonded at its side portion and bottom portion to form a cylindrical structure having a closed bottom end. This conventional method of producing the separator base paper 21 must be turned at right angle to change its moving direction from the cutting step to a winding step in which the cut base paper is wound and adhered to form a cylindrical structure and its production rate is limited at most to 100 pieces per minute.
Therefore, if it is necessary to produce about 600 separators 4 per minute, as shown in FIG. 3, a roll 20 of a separator base paper 21 is rotated in the horizontal direction to pull the base paper in the longitudinal (horizontal) direction and then cut into a predetermined length, and the cut base paper is rotated in the horizontal direction by using a mandrel wound into a cylindrical form, and then the cylindrical base paper is heat-bonded at its side and bottom end portions so that a cylindrical structure with a closed bottom end is formed for a cylindrical separator. This method does not require any change of moving direction of the base paper 21 from the cutting step to the winding step and, consequently, the production can be enhanced.
However, in the production method of FIG. 3 in which the base paper 21 is rotated in the horizontal direction and pulled in the longitudinal direction to be cut and then rotated in the horizontal direction, an axial direction of the separator 4 is coincided with a width direction of the base plate 21 of the roll 20 and, therefore, the cylindrically formed separator is likely to be deformed when the end 4A (FIG. 1) of the separator 4 is pressed radially inwardly by pushing the end 4A by the gasket 9 of the closure body 6. If such deformation is generated, the gel like zinc anode active mass 5 is moved into the cathode active mass 3 through a gap between the separator 4 and the gasket 9 when a shock or vibration is added to the alkaline cell 1, resulting in generation of short circuits.
An example of the production method of the separator shown and described with reference to FIG. 3 will be described more specifically with reference to FIGS. 4A, 4B, 4C and 4D. In FIG. 4A, a base paper 21 in the form of a roll 20 is cut into a predetermined dimension and the cut base paper 21 is wound to form a cylindrical body 22 and then directed to a bottom forming step by a suitable feeding device (not shown) and one end of the cylindrical body is subject to a heat bonding to provide a closed bottom end.
In order to provide a reliable sealing effect of the closed bottom end, the end portion of the cylindrical body is bent prior to the heat bonding. Namely, as shown in FIG. 4B, a shaft 25 is inserted through the cylindrical body 22 and a blade-like die 24 is pressed downward against the upper end of the cylindrical body 22 to form a depression 22b as shown in FIG. 4C, and then a heat curl die 29 is pressed against the depressed upper end of the cylindrical body 22 as shown in FIG. 4D so that a semispherical end 22a is formed as illustrated in FIG. 4E. Thus, the conventional separator 4 was obtained.
However, there are some inconveniences and difficulties in the conventional production method of the separator.
First, the cylindrical body 22 must be fed to a predetermined position by means of a feeder and, therefore, it is likely that the opening end portion of the cylindrical body 22 is accidentally deformed or collapsed during the feeding transporting operation due to some mechanical factors and/or shocks applied to the cylindrical body 22, resulting in serious reduction of productivity.
Second, when a depression 22b is formed on top of the cylindrical body 22, it depends largely upon the physical properties, as hardness, of the cylindrical body 22 and there are cases that it is difficult to form a desirable depression and, therefore, a sealing (blocking) property of the closed bottom end 22b is not complete or satisfactory. If a depression 22b is forcibly formed by adjusting the time and pressure of the blade-like die 24 against the top end of the cylindrical body 22, it sometimes results in damages of the cylindrical body and reduction of productivity. Thus, a care must be taken to select definitely the material for the base paper for the separator but this is troublesome and requires additional time and labor.